300,000 individuals sustain sports concussions each year in the U.S. The majority of at-risk athletes are at the high school level. High school football players represent a particularly high-risk group, accounting for about two-thirds of all concussions in high school athletes. The short- and long-term effects of sports concussions and perhaps more importantly repetitive sub-concussive impacts are not known, nor are the cumulative effects of repeated injuries understood. There is enormous variability in outcome, although the reasons for this variability are not understood. We propose to use technological advances in on-field head impact monitoring, cognitive testing, and functional brain imaging to learn for the first time what types of head impacts, under what circumstances, in which individuals, cause what effects in brain function. This proposal will help to determine the short and long term effects of repetitive biomechanical force exposure on the developing adolescent brain, provide important information on the biomechanics of sports-related traumatic brain injury, and lead to more informed return-to-play guidelines. The overarching theme of this proposal is that a youth's exposure to biomechanical forces is a critical factor influencing cognitive outcome. This has two broad components: the characteristics of a single impact (e.g. linear acceleration, rotational acceleration, direction etc.), and the history of exposure to biomechanical forces over time (e.g. measures of frequency and intensity of impacts over the preceding days and weeks). We propose to use technological advances in on-field head impact monitoring, cognitive testing, and functional brain imaging to learn for the first time what types of head impacts, under what circumstances, in which individuals, cause what effects in brain function. Exposure to biomechanical forces acutely (post-concussive), sub-acutely (pre- to one-month post-season), and cumulatively (exposure over multiple seasons of play) are of interest and will be monitored. Two groups of high school student athletes (football players and non-impact sport athletes) will be studied at three time points (preseason, after a single season of play, and after multiple seasons of play) using a standardized cognitive battery and functional MRI (fMRI). A subgroup of students who sustain concussion, and matched controls from their team and from the non-impact athlete group will also be studied within one week of the concussion. Impact parameters will be directly measured using helmet-based accelerometer units.